Thyristor Commutation Techniques

Introduction to Thyristor Commutation

What is Thyristor Commutation?

Definition

Commutation is the process of turning OFF a conducting thyristor by reducing the anode current below the holding current level.

Key Characteristics:

  • Thyristors can be turned ON by gate signal but cannot be turned OFF by gate signal

  • Once triggered ON, they remain conducting until commutated

  • Commutation is essential for controlled switching in power circuits

  • Different techniques are used for different applications

Importance of Commutation

Why is commutation critical?

Control:

Enables precise control of power flow in circuits

Protection:

Prevents damage from overcurrent conditions

Circuit Operation:

Essential for proper functioning of converters

Flexibility:

Allows various circuit configurations and applications

Key Point

Without proper commutation, thyristors cannot be used effectively in many power electronic applications.

Classification of Commutation Techniques

Commutation Methods Overview

Two Main Categories

  1. Natural Commutation

  2. Forced Commutation

Natural Commutation

  • Line commutation

  • Load commutation

Uses AC supply characteristics

Forced Commutation

  • Class A (Resonant)

  • Class B (LC Circuit)

  • Class C (Complementary)

  • Class D (Auxiliary)

  • Class E (External Pulse)

Uses external circuits

Natural Commutation

Natural Commutation Principles

Definition

Natural commutation occurs when the anode current naturally becomes zero due to AC supply conditions, without any external commutation circuit.

Operating Principle:

  • Makes use of the alternating nature of AC voltages

  • Current passes through zero every half cycle in AC circuits

  • When current becomes zero, reverse voltage appears across the thyristor

  • This reverse voltage turns OFF the thyristor

Applications

AC voltage controllers, rectifiers, cycloconverters

Types of Natural Commutation

  1. Line Commutation:

    • Current zero occurs due to AC supply voltage reversal

    • Most common in rectifier circuits

    • Thyristor turns OFF when AC supply goes negative

  2. Load Commutation:

    • Current zero occurs due to load circuit characteristics

    • Load impedance causes current to become zero

    • Less common than line commutation

Limitation

Only works with AC supply or oscillatory currents

Forced Commutation Techniques

Forced Commutation Overview

When is Forced Commutation Required?

  • DC circuits (choppers)

  • Inductive load circuits

  • Inverter circuits

  • When controlled turn-OFF is needed

Basic Principle:

  1. Force the anode current below holding current

  2. Apply reverse voltage across thyristor

  3. Provide sufficient turn-off time (\(t_q\))

Class A - Resonant Commutation

Class A: Resonant Commutation circuit & Operation

Characteristics:

  • Also known as self commutation

  • Uses L-C components to create oscillatory current

  • Current oscillates and passes through zero

  • Also called current commutation method

Class A commutation circuit
Class A commutation circuit

Key Features:

  • Load forms part of resonant circuit

  • Current has sinusoidal/oscillatory nature

  • Automatic turn-OFF at current zero

Applications

High frequency applications (above 1000 Hz), series inverter circuits, oscillator circuits

Class A: Operating Principle

Circuit Operation Sequence:

  1. When thyristor is triggered, L-C-R circuit is excited by DC source

  2. Current has oscillatory/sinusoidal nature due to L-C resonance

  3. Current decreases to zero at some point

  4. Thyristor automatically turns OFF at current zero

  5. Beyond zero point, current tries to reverse but thyristor blocks it

Advantage

Simple and automatic commutation without external control

Class A: Voltage and Current Waveforms

Voltages and currents in Class A (load is parallel with capacitor)
Voltages and currents in Class A (load is parallel with capacitor)

Class B - LC Circuit Commutation

Class B: LC Circuit Commutation

Circuit Configuration:

  • LC resonant circuit is connected across the thyristor

  • Capacitor is initially charged to supply voltage

  • Load is separate from commutating circuit

Class B commutation circuit and associated waveforms
Class B commutation circuit and associated waveforms

Operation Principle:

  • When thyristor is triggered, two currents flow:

    • Load current through load

    • Commutating current through LC circuit

  • Oscillatory current opposes load current

  • When commutating current exceeds load current, net current becomes zero

Class B: Design Requirements

Critical Design Requirement:

\[\text{Peak commutating current} > \text{Load current}\]

For successful commutation:

\[I_C = \dfrac{V_C}{\sqrt{L/C}} \geq I_L\]

Turn-off time provided:

\[t_{off} = \dfrac{\pi}{2}\sqrt{LC}\]

Applications

DC chopper circuits, applications where load and commutating circuits need to be separate

Class C - Complementary Commutation

Class C: Complementary Commutation Circuit & Operation

Circuit Elements:

  • Main thyristor (\(T_1\)) in series with load

  • Auxiliary thyristor (\(T_2\)) - complementary thyristor

  • Commutating capacitor (C)

  • Both thyristors share the same load current path

Class C–commutation circuit
Class C–commutation circuit

Key Features:

  • Uses pre-charged capacitor for commutation

  • Very reliable commutation method

  • Suitable for frequencies below 1000 Hz

  • Two thyristors operate alternately

Class C: Circuit Operation

Mode 1: \(T_1\) is triggered and conducting

  • Load current flows through \(T_1\)

  • Capacitor C charges to supply voltage through load and \(T_1\)

  • \(T_2\) is OFF

Mode 2: \(T_2\) is triggered for commutation

  • \(T_2\) turns ON

  • Charged capacitor applies reverse voltage to \(T_1\)

  • \(T_1\) gets reverse biased and turns OFF

  • Load current now flows through \(T_2\)

  • Capacitor voltage reverses

Result: \(T_1\) is successfully turned OFF and \(T_2\) takes over

Class C: Design Parameters

Turn-off time:

\[t_{off} = RC \ln(2) \approx 0.69 RC\]

Capacitor Selection:

\[C = \dfrac{1.44 \times t_{off}}{R}\]
where \(t_{off} \geq t_q\) (thyristor turn-off time)

Applications

McMurray-Bedford inverter, AC motor drives, applications requiring reliable commutation

Class C: Voltage and Current Waveforms

Voltages and currents in Class C
Voltages and currents in Class C

Class D - Auxiliary Commutation

Class D: Auxiliary Thyristor Commutation

Circuit Components:

  • Main thyristor (\(T_1\)) with load

  • Auxiliary thyristor (\(T_2\)) for commutation

  • Commutating inductor (L) and capacitor (C)

  • Diode (D) for proper circuit operation

Key Features:

  • Separate commutation circuit

  • Can transfer commutation energy back to load

  • Higher efficiency possible

  • More complex than Class C

Class D–commutation circuit
Class D–commutation circuit

Class D: Operation Sequence

Step 1: Capacitor charging

  • \(T_2\) is triggered first

  • Capacitor C charges through: Supply \(\to\) C \(\to\) \(T_2\) \(\to\) Load

  • When fully charged, \(T_2\) turns OFF naturally

Step 2: Main thyristor operation

  • \(T_1\) is triggered, load current flows through \(T_1\)

  • Capacitor voltage reverses during discharge

Step 3: Commutation

  • To turn OFF \(T_1\), trigger \(T_2\) again

  • Reverse charged capacitor applies reverse voltage to \(T_1\)

Class D: Design Considerations

Basic Design Relations:

For turn-off time:

\[t_{off} \approx \sqrt{LC}\]

For successful commutation:

\[\text{Peak capacitor current} > \text{Load current}\]

Applications

Jones chopper circuit, DC motor drives, applications requiring energy recovery

Class D: Voltage and Current Waveforms

Voltages and currents in Class D
Voltages and currents in Class D

Class E - External Pulse Commutation

Class E: External Pulse Commutation Circuit & Operation

Principle:

  • Uses external pulse source for commutation

  • Pulse transformer provides isolation

  • Reverse voltage pulse applied to thyristor

  • Duration of pulse must exceed turn-off time

Class E–commutation circuit
Class E–commutation circuit

Advantages:

  • Very efficient

  • Simple control

  • Fast commutation

Disadvantages:

  • Additional pulse source

  • Critical transformer design

  • Less commonly used

Comparison and Selection

Comparison of Commutation Methods

Comparison of commutation techniques
Parameter Natural Class A Class B Class C Class D
Circuit Complexity Low Medium Medium Medium High
Extra Components None L, C L, C C, SCR L, C, SCR, D
Reliability High Medium High High High
Efficiency High Medium Medium Medium High
Cost Low Low Medium Medium High
Applications AC circuits Oscillators Choppers Inverters Choppers

Selection Guidelines

Choose Natural Commutation when:

  • AC supply is available

  • Simple circuit is desired

  • Cost is a major factor

Choose Forced Commutation when:

  • DC supply is used

  • Controlled turn-OFF is needed

  • Inverter or chopper application

Key Selection Factors: Power level, load type, operating frequency, cost constraints, efficiency requirements

Applications in Power Electronics

Practical Applications

Natural Commutation:

  • AC voltage controllers

  • Phase-controlled rectifiers

  • Cycloconverters

  • AC motor soft starters

Class A Commutation:

  • Series resonant inverters

  • High-frequency oscillators

  • Induction heating

Class B & D:

  • DC choppers

  • DC motor drives

  • Battery chargers

  • SMPS circuits

Class C:

  • Voltage source inverters

  • UPS systems

  • AC motor drives

Design Example

Practical Design Example: Class B Commutation

Given Parameters:

  • Load current: \(I_L = 5\) A

  • Supply voltage: \(V_s = 50\) V

  • Required turn-off time: \(t_{off} = 50 \mu s\)

Design Approach:

From turn-off time requirement:

\[t_{off} = \dfrac{\pi}{2}\sqrt{LC} = 50 \times 10^{-6} \text{ s}\]

Therefore: \(\sqrt{LC} = 31.8 \times 10^{-6}\) s

For successful commutation (safety factor 1.5):

\[I_{peak} = \dfrac{V_C}{\sqrt{L/C}} \geq 1.5 \times 5 = 7.5 \text{ A}\]

Design Example: Solution

Solving the Design equations:

Given constraints:

\[\begin{aligned} \sqrt{LC} &= 31.8 \times 10^{-6} \text{ s} \\ \sqrt{L/C} &\leq 6.67 \Omega \text{ (for } V_C = 50\text{ V)} \end{aligned}\]

Solution:

  • Inductor: \(L = 31.8 \times 10^{-6} \times 6.67 = 212 \mu H\)

  • Capacitor: \(C = \dfrac{31.8 \times 10^{-6}}{6.67} = 4.77 \mu F\)

Standard Values:

  • \(L = 220 \mu H\)

  • \(C = 4.7 \mu F\)

Verification

Turn-off time \(\approx 50 \mu s~\checkmark\) , Peak current \(> 7.5 A~\checkmark\)

Summary and Conclusion

Key Learning Points

Essential Concepts:

  1. Commutation is essential for turning OFF thyristors

  2. Natural commutation works only with AC supply

  3. Forced commutation is needed for DC circuits and controlled applications

  4. Different forced commutation classes suit different applications

  5. Proper design ensures reliable thyristor operation

  6. Selection depends on application requirements and constraints

Important Takeaway

Understanding commutation techniques is fundamental for designing thyristor-based power electronic circuits.

Final Remarks

Design Considerations:

  • Always ensure sufficient turn-off time (\(t_{off} \geq t_q\))

  • Consider safety margins in current ratings

  • Account for component tolerances

  • Verify thermal and voltage ratings

Future Trends:

  • Modern power devices (IGBTs, MOSFETs) offer gate turn-off capability

  • Thyristors still important for high-power, high-voltage applications

  • Understanding principles remains valuable for power electronics